Riess and Schmidt were lead investigators on the High-Z Supernova Search collaboration, while Perlmutter was with the Supernova Cosmology Project. Back in 1998, both teams published papers indicating a remarkable discovery: the rate at which the universe is expanding is increasing, a result completely at odds with established cosmological models of the day. The fact that this conclusion was arrived at independently by two competing teams made the astrophysics community far more receptive to the otherwise outlandish result, and subsequent observations by other teams have served to validate the work. These observations have also given rise to one of the most bewildering and mysterious concepts in modern physics: dark energy (not to be confused with dark matter, which is another topic altogether).

When Einstein first formulated his field equations for General Relativity, it was quickly realized (based upon the work of Alexander Friedman) that all reasonable solutions for them implied an expanding universe. At the time, Hubble had not yet discovered the cosmological redshift proving an expanding universe, and Monsignor George Lemaître had not yet constructed his Big Bang Theory. To get around this seeming problem and make his theory consistent with the concept of a static universe (as it was perceived at the time), Einstein added a term to his field equations containing what he dubbed the Cosmological Constant. Later, when the universe was indeed shown to be expanding, Einstein ripped out this addition, dubbing his failure to use General Relativity to predict cosmic expansion the biggest blunder of his career.

Determining how fast the universe is expanding is a tricky business. Two pieces of data are necessary to yield an accurate value: the redshift of the spectrum of the receding object, which is simple enough to measure to a great degree of accuracy, and the distance to the object. The latter is not so easy to measure in practice, and, for distant objects such as galaxies, astronomers are forced to rely upon a ladder of measurements based upon the concept of a “standard candle,” an object whose luminosity is known, and thus its brightness can be used to measure its distance. Back in 1974, Kirshner and Kwan proposed using Type 1a supernovae as a standard candle for measuring extra-galactic distances (since such supernovae follow a consistent and predictable luminosity profile as they evolve), and in 1977 Bob Wagoner showed how to use such observations to determine the rate at which cosmic expansion is changing by comparing observations of extra-galactic supernovae and various distances.

Fast forward to the late 90’s. With the Hubble Space Telescope in place, astronomers finally had a tool in place for applying these ideas with unrivaled precision. Which leads us to the announcements in 1998 (which in turn gave rise to today’s Nobel Prize announcement):

Explaining these results has presented a huge problem for cosmologists. They had by and large expected cosmic expansion to be slowing, or at the very least staying the same. Accelerating expansion was quite unexpected. But then, it is always the unexpected results which yield interesting discoveries. Some have resorted to resurrecting Einstein’s Cosmological Constant (but giving it a different sign to force acceleration rather than keeping expansion in check). Others have posited variants of this. All imply some sort of phenomenon which has come to be known as dark energy, a sort of ambient vacuum energy permeating spacetime and driving the acceleration of the Hubble expansion (not to be confused with, but perhaps related to, the quantum vacuum energy detectable via the Casimir effect). In any event, the discovery has provided physicists with mysteries to ponder for quite some time.